Subtract the following rational expressions. You may keep the denominator in its factored form. $\dfrac{5}{7k-9}-\dfrac{2k}{3k-21}=$
Answer: We can subtract two rational expressions whose denominators are equal by subtracting the numerators and keeping the denominator the same. When the denominators are not the same, we must manipulate them so that they become the same. More specifically, we should find the least common multiple of the two denominators. [What's that?] To do that, we first need to factor the denominators as much as possible: The first denominator ${(7k-9)}$ cannot be factored further. The second denominator $3k-21$ can be factored as ${(3)(k-7)}$ by factoring out a $3$. We can see that: Only the first denominator has the factor ${(7k-9)}$. Only the second denominator has the factors ${(3)(k-7)}$. Therefore, the least common multiple is the product of all the above factors. Let's manipulate the expressions to have that denominator: $\begin{aligned}&\phantom{=}\dfrac{5}{{(7k-9)}}-\dfrac{2k}{{(3)(k-7)}}\\\\\\\\ &=\dfrac{5{(3)(k-7)}}{{(7k-9)}{(3)(k-7)}}-\dfrac{2k{(7k-9)}}{{(3)(k-7)}{(7k-9)}}\\\\\\\\ &=\dfrac{15k-105}{3(k-7)(7k-9)}-\dfrac{14k^2-18k}{3(k-7)(7k-9)}\end{aligned}$ Now that both denominators are the same, let's subtract! $\phantom{=}\dfrac{15k-105}{3(k-7)(7k-9)}-\dfrac{14k^2-18k}{3(k-7)(7k-9)}$ $\begin{aligned} &=\dfrac{(15k-105)-(14k^2-18k)}{3(k-7)(7k-9)}&\text{Subtract numerators}\\\\\\\\ &=\dfrac{15k-105-14k^2+18k}{3(k-7)(7k-9)}&\text{Distribute}\\\\\\\\ &=\dfrac{-14k^2+33k-105}{3(k-7)(7k-9)}&\text{Combine like terms}\end{aligned}$ In conclusion, $\dfrac{5}{7k-9}-\dfrac{2k}{3k-21}=\dfrac{-14k^2+33k-105}{3(k-7)(7k-9)}$